1. Field of the Invention
This invention relates to growing crystalline films on substrate materials, and more particularly concerns a method and apparatus for forming epitaxial films.
2. Description of Related Art
Infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having wavelengths between 1-15 .mu.m. These detectors often operate on the principle of photoconductivity, in which infrared radiation changes the electrical conductivity of the material upon which the radiation is incident. Elemental detectors may also rely on photovoltaic or photoemission properties of certain materials to detect infrared radiation.
To fabricate such detectors, crystalline films of materials such as Hg.sub.1-x Cd.sub.x Te are often grown on substrate materials which may have an initial layer of CdTe. Techniques for growing such films include liquid-phase epitaxy ("LPE"). In LPE technology, a crystalline film is formed by using a "bottomless bin" filled with a melt which is either at solid-liquid equilibrium or slightly undersaturated. The melt is then cooled while a substrate material is exposed to the bottom of the bin causing the film to grow on the substrate. The desired thickness of the film is obtained by controlling the temperature of the melt or the duration of the exposure of the substrate to the melt. Other techniques for forming epitaxial films include chemical vapor deposition and molecular beam epitaxy.
While such methods for epitaxially growing films on substrate materials are generally effective, they often have several disadvantages. The films could generally be grown on a relatively small number of substrates simultaneously and often did not produce films of relatively uniform thickness on each substrate. Annealing was also required to fill vacancies remaining in the grown film after growth had occurred to insure the desired electrical characteristics. Further, the films produced by these technologies tended to have relatively high interface state densities which would generally reduce charge transfer efficiency. Impurities were also introduced into the films when certain growth techniques were used as the melt, which often contained impurities, was often required to be placed in direct contact with the substrate. These impurities would often trap charge carriers which also tended to reduce charge transfer efficiency. In addition, since these methods often required the use of compounded sources such as HgTe or Hg.sub.x Te.sub.1-x, impurities introduced into the sources during the compounding process would also be present in the resulting films. Finally, these processes were not generally used to dope the films during the growth, nor were they generally used in conjunction with masks to grow films on selected regions of the substrate.